Division of Cardiology, Children’s Hospital of Philadelphia, Department of Pediatrics and Center for Clinical Epidemiology and Biostatistics Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA

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To the Editor:

We read with interest the recent article by Johnson et al1 describing cumulative radiation exposure and estimates of lifetime attributable risk (LAR) of cancer for a cohort of children ≤6 years of age undergoing 1 of 7 primary surgical procedures at their institution. With a wide range of exposures across the cohort, the authors report an overall median cumulative exposure of 2.7 mSv, the majority from cardiac catheterization. Similarly, a wide range of estimated LAR was reported. This article represents another valuable contribution to the literature on the important topic of cumulative radiation exposure from medical testing for pediatric patients with chronic medical conditions, such as congenital heart disease.

However, we feel compelled to add a note of caution on applying such estimates of LAR to an individual patient. This caution derives primarily from the potential for variability in the measures underlying these risk calculations. As the authors acknowledge, there is a wide range in the radiation exposure for patients treated for the same heart disease at the same institution. That range is likely magnified when comparisons are made between institutions. We previously described cumulative exposure to medical sources of radiation for a large cohort of patients undergoing congenital heart surgery.2 Comparing lesion-by-lesion radiation dose estimates with the present report reveals a number of disparities. This is undoubtedly because of differences in the method of dose measurement, equipment used, operator techniques, practice patterns, and time period. We have also recently reported cumulative radiation exposure using examination-specific doses from the 2 subgroups with the highest exposures, children after a Norwood operation3 and heart transplantation.4 Again, substantial disparities are seen in comparing radiation doses with the current report. The point is not that one is right and the other is wrong, the point is simply that doses are different, as would be expected across institutions.

Further potential for imprecision comes from the method used to translate measured dose into LAR. Although the methodology outlined in Biological Effects of Ionizing Radiation VII5 is the best available, a number of assumptions are required that have the potential to introduce error, notably that all radiation exposure occurs at age 5 years and the expected duration of a “lifetime.” In addition, there is the possibility that the linear no-threshold model, which underlies the LAR estimation, may not be accurate for low levels of exposure.6 Finally, this estimation also ignores fractionation, the notion that small repeated exposures have less deleterious consequences than the same total dose given at once. In our opinion, all of these potential sources of variability and imprecision make it potentially misleading to claim “the estimated LAR of cancer above baseline was as high as 6.5%,”1 particularly when this number represents the 95% confidence limit of the estimate from the 10 subjects in this cohort who had a cardiac transplantation. We applaud the authors’ efforts in shining a brighter light on this important issue, but wonder whether the estimates reported for LAR will provide substantiated value in caring for our next patient with congenital heart disease.

Andrew C. Glatz, MD, MSCEDivision of CardiologyChildren’s Hospital of PhiladelphiaDepartment of Pediatrics and Center for Clinical Epidemiology and Biostatistics Perelman School of MedicineUniversity of PennsylvaniaPhiladelphia, PA

Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation; National Research Council of the National Academies.Health Risks From Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2. Washington, DC: The National Academies Press; 2006.